Abstract
Mackerel caught in the North Sea (130), Skagerrak (148) and southwest of Ireland (176) were examined for ectoparasites. The most common parasites were Kuhnia scombri and K. sprostonae (Monogenea), and Caligus pelamydis (Copepoda). The prevalence of K. scombri decreased as the age of the host increased, from about 75 % in the age group 1–3 years to c. 40 % in 7–10 year-old fish. K. sprostonae showed the same trend, but its prevalence ranged between zero and 22 %. Most fish carried from 1 to 5 K. scombri, but up to 103 monogeneans were recorded on a single host. Kuhnia scombri preferred the middle third of the first and second gill arches and the base of the gill filaments, whereas most K. sprostonae were attached to the pseudobranchs. Only one Caligus elongatus was detected in the Skagerrak material whereas 45 % of 7–10 year-old mackerel from Ireland were infected. There was no relationship between the age of the host and the intensity of infection. Caligus pelamydis was found on the inner surface of the gill cover and most fishes carried only a single specimen. More than half the C. pelamydis from Ireland carried epizoic peritrich ciliates.
The monogenean Grubea cochlear (6 specimens) was found on two fish from the Skagerrak (new locality), the copepods Advena paradoxa (7 specimens) on hosts from Ireland and the Skagerrak (new localities), Caligus elongatus (2 specimens) in Ireland and the North Sea, Clavellisa scombri (16 specimens) in all localities, thus confirming its presence on Atlantic mackerel, and a single Peniculus sp., probably a new species, on a host from Ireland. No ectoparasites were found to be valuable as biological tags, but we discuss whether variations in the length of hamuli from K. scombri in fish from different stocks indicate that there are also discrete populations of the parasite. Most mackerel carried unidentified cysts on the gills.
